High viscosity gelled hydrocarbon fluids provide unique functional characteristics for a variety of applications. Although these fluids have a high viscosity, they are capable of breaking into thinner fluids under predetermined conditions. The oil industry uses these fluids in many day-to-day operations. Common applications include: fracturing subterranean formations penetrated by a well bore, forming gravel packs in a downhole environment, performing pigging operations in a pipeline and other applications requiring a viscous fluid. Although high in initial viscosity, these fluids are designed to subsequently breakdown into thinner fluids. The subsequent drop in fluid viscosity is necessary to permit production of hydrocarbons from the well.
Efficient formulation of gelled or viscosified hydrocarbon liquids requires prior knowledge of the viscosity of the liquid under the intended operating conditions. Due to difficulties in accurately determining rheology under downhole conditions, most well operators estimate the concentration of viscosity breaking agents necessary to lower, i.e. break, the viscosity of the gelled fluid using tests without the presence of liquid carbon dioxide. If the operator underestimates the concentration required to lower the viscosity, then production may be permanently impaired due to an unbroken gel. Absent the presence of viscosity breaking agents, oil gels will not break on their own accord over time. Accordingly, most operators add an excess of viscosity breaker to ensure an adequate drop in viscosity. However, excess use of viscosity breaker will prematurely degrade the viscosity of the fluid. This could result in a failure of the treatment to properly place the proppant in the fracture. Additionally, excess breaker usage unnecessarily increases costs. Clearly, the ability to accurately formulate the downhole fluid will greatly improve operational efficiency and reduce costs by eliminating over use of the viscosity breaking agents and other additives. Therefore, a method for determining the rheology of the gelled or viscosified hydrocarbon liquid under operational conditions will enhance the current state of the art.
The rotary viscometer is a tool commonly used to determine rheology of gelled hydrocarbon liquids. In general, a rotary viscometer includes a sample chamber, a rotary sleeve and a static bob coaxially positioned within the rotary sleeve. Depending upon the sample to be tested, the sample chamber may also include a heater.
When assessing the rheology of a fluid, the operator fills the sample chamber with sufficient fluid to cover a predetermined portion of the bob. Typically, a computer-controlled motor rotates the sleeve thereby shearing the fluid and transmitting torque via the fluid to the bob. The bob deflects due to the applied torque. Alternatively, one may measure the force required to maintain the bob in the stationary position or the power required to maintain the sleeve at a constant rotational speed. In general, the basic procedures for determining fluid rheology are well known in the art.
When using a standard sleeve and bob arrangement, measurement of the resulting angular displacement permits calculation of the shear stress experienced by the fluid. Additionally, the shear rate may be calculated by using the rotational speed and the known clearance distance between the sleeve and the bob. As known to those skilled in the art, apparent viscosity is determined by dividing the measured Shear Stress by the calculated Shear Rate. Preferably, standard sensors and instrumentation perform all such measurements and calculations automatically.
Most rotary viscometers are top driven units, i.e. the driving force for the sleeve is located above the sleeve. Therefore, current methods of using rotary viscometers require an air gap or void space separating the upper surface of the sample from the upper level of the sample chamber. The void space, filled with air or other undissolved gaseous material, acts as a buffer zone precluding entry of the sample into the bearings, drive system and sensing mechanism of the viscometer. Intrusion of the sample into these components is deleterious to the operation of the viscometer. At best, entry of the gelled fluid into the bearings increases drag and produces incorrect readings.
The risk of contamination increases when working with non-Newtonian fluids, as these fluids commonly exhibit the Weissenberg effect. The Weissenberg effect describes the tendency of a non-Newtonian fluid to climb up a rotating rod. When working with non-Newtonian fluids, the operator must take care not to overfill the sample chamber otherwise the non-Newtonian fluid will climb the shaft rotating the sleeve and contaminate the bearing and drive system. Therefore, it would be desirable to provide a method for determining the rheology of non-Newtonian fluids without the detrimental impact of the Weissenberg effect.
The current invention provides a method for determining the rheology of a mixture of gelled hydrocarbon fluid and liquid carbon dioxide. The method utilizes a bottom driven rotary viscometer, also known as a rheometer. The rotary viscometer has a sample chamber for holding the fluid to be examined. The sample chamber includes a rotary sleeve driven from below. Positioned coaxially within the rotary sleeve is a static bob. The static bob is connected to a device for measuring torque applied to the bob. Preferably, the device also calculates the apparent viscosity of the fluid sample. The sample chamber includes multiple ports for adding fluids to the chamber. Additionally, the sample chamber may be pressurized to a pressure ranging from about 3 MPa to about 103 MPa and heated to about 205xc2x0 C. In practice, the chamber is typically operated at temperatures between about 0xc2x0 C. and 175xc2x0 C. Typical operating pressures will range from about 17 MPa to about 103 MPa. To ensure a homogeneous mixture of gelled hydrocarbon fluid and liquid carbon dioxide, the sample chamber includes a means for mixing the fluids within the sample chamber. One means for mixing the fluids is the addition of fins to the external walls of the rotary sleeve. Alternatively, the rotary sleeve may have grooves machined into or otherwise incorporated into the outer surface thereof to promote mixing. Thus, rotation of the sleeve provides the means for mixing the fluids.
According to the method of the current invention, a sample chamber located in a bottom driven rotary viscometer is filled with gelled hydrocarbon and liquid carbon dioxide in a manner to substantially fill the sample chamber thereby eliminating substantially all undissolved gases from the interior of the sample chamber. Any gas remaining in the sample chamber is likely carbon dioxide. Subsequently, the liquid carbon dioxide and gelled hydrocarbon are mixed to form a homogenous mixture. The components of the mixture are placed under pressure and optionally heated. Once a homogenous mixture has been formed, a drive shaft or other suitable means rotates a sleeve positioned within the sample chamber. The drive shaft is located beneath the sleeve. Sleeve rotation shears the fluid mixture and transmits torque via the mixture to a bob positioned coaxially within the sleeve. Torque measurements are taken and used to determine the rheology of the mixture within the sample chamber.
The current invention also provides a method for determining the rheology of a mixture of gelled hydrocarbon and liquid carbon dioxide at temperatures lower than room temperature. According to the method of the current invention, a sample chamber located in a bottom driven rotary viscometer is filled with gelled hydrocarbon and liquid carbon dioxide in a manner to substantially eliminate all undissolved gases from the interior of the sample chamber. Subsequently, the liquid carbon dioxide and gelled hydrocarbon are mixed to form a homogenous mixture. Before, during or after the mixing, the components of the mixture are cooled and placed under pressure. Once a homogenous mixture has been formed, a drive shaft or other suitable means rotates a sleeve positioned within the sample chamber. The drive shaft is located beneath the sleeve. Rotation of the sleeve shears the fluid mixture and transmits torque via the mixture to a bob positioned coaxially within the sleeve. Torque measurements are taken and used to determine the rheology of the mixture within the sample chamber.
The current invention also provides a method for precluding the Weissenberg effect when measuring the rheology of non-Newtonian fluids with a rotary viscometer. The method comprises the steps of filling a sample chamber located within a bottom driven rotary viscometer with a non-Newtonian fluid. After the sample chamber has been filled to a point where substantially all undissolved gases have been eliminated from the sample chamber, a drive shaft or other suitable means rotates a sleeve positioned within the chamber. The drive shaft is located beneath the sleeve. As the sleeve rotates, the non-Newtonian fluid shears and transmits torque to a static bob positioned coaxially within the sleeve. Torque exerted on the bob is measured and used to determine the rheology of the non-Newtonian fluid.